(a) Field of the Invention
The present invention relates to an exhaust gas treatment method and treatment facilities that remove a sulfuric acid mist in an exhaust gas by a wet type electrostatic precipitator.
(b) Description of the Related Arts
Exhaust gases given off from boilers, combustion furnaces and the like using petroleum, heavy oil and the like as fuel contain sulfur oxides such as sulfur dioxide and sulfur trioxide. Particularly, a heavy oil exhaust gas containing sulfur components in high concentrations has high content of SO3 in the exhaust gas.
In a conventional exhaust gas treatment plant, a dry type electrostatic precipitator and a wet type flue gas desulfurization equipment are disposed in this order subsequent to a boiler. An exhaust gas discharged from the boiler is dusted by the dry type electrostatic precipitator before being introduced into the wet type flue gas desulfurization equipment. In the wet type flue gas desulfurization equipment, a Ca base or Na base agent is used as an alkali agent to remove sulfur oxide in the exhaust gas. SO3 of sulfur oxides has a small particle size and is in a mist state. Thus, SO3 cannot be adequately removed by the wet type flue gas desulfurization equipment and therefore, a wet type electrostatic precipitator is separately provided subsequent to the wet type flue gas desulfurization equipment to remove SO3.
An exhaust gas treatment plant according to Japanese Patent Application Laid-Open No. 2007-260619 determines the amount of sulfuric acid mist after wet type desulfurization and mixes an ammonia gas whose equivalent weight ratio to the determined sulfuric acid mist is 1 or less between the wet type flue gas desulfurization equipment and the wet type electrostatic precipitator to remove the sulfuric acid mist by the wet type electrostatic precipitator.
However, a mist introduced into the wet type electrostatic precipitator has SO3 as a main component and pH thereof is smaller than 2 and thus, the inside of the wet type electrostatic precipitator is subject to violent corrosion. Thus, measures to slow down the corrosion speed by spraying spray water whose pH is adjusted into the wet type electrostatic precipitator continuously or the like are taken. Also, a heat sinking unit of a gas-gas heater subsequent to the electrostatic precipitator has a problem of shortened life of material due to a sulfuric acid mist sticking to the surface of a pipe group and concentrated by an evaporation process.
As shown in Japanese Patent Application Laid-Open No. 2007-260619, equipment such as a compressed gas cylinder of ammonia and a pressure control valve is needed to feed an ammonia gas to allow a reaction with a sulfuric acid mist into the plant and such equipment has a problem of complicated handling.
To solve the problems of the above conventional technology, an object of a treatment method and a treatment plant of an exhaust gas according to the present invention is to provide a treatment method and a plant of an exhaust gas in which corrosion resistance of the exhaust gas treatment plant to a sulfuric acid mist after wet type desulfurization is increased. Also, an object of a treatment method and a plant of an exhaust gas according to the present invention is to make the feeding of ammonia allowed to react with a sulfuric acid mist easier.